Send mail from the shared mailbox

  1. Open Outlook.

  2. Choose New Email.

  3. If you don't see the From field at the top of your message, choose Options > From.

  4. Click From in the message, and change to the shared email address. If you don't see your shared email address, choose Other email address and then type in the shared email address.

  5. Choose OK.

  6. Finish typing your message and then choose Send.

From now on when you want to send a message from your shared mailbox, the address will be available in your From drop down list. Whenever you send a message from your shared mailbox, your recipients will only see the shared email address in the message.

Source: Open and use a shared mailbox in Outlook - Microsoft Support